It is desirable to be able to monitor particles in a fluid flowing through a conduit. It is particularly desirable to be able to monitor formation sand that is entrained in oil and gas production flow streams. In such systems, the particles may influence erosion, erosion/corrosion and/or corrosion of the conduit surface, potentially leading to breaches of the conduit by the fluid. Therefore, it is important to be able to measure the metal loss of the conduit surface, especially at conduit bends where the metal loss rate is greatest. Nonetheless, it is also important to monitor metal loss on straight sections of a conduit. (It should be noted that, in the art, “corrosion” is a broad term encompassing all types of surface metal loss, including erosion and erosion/corrosion.)
Conventionally, the corrosion of such a conduit surface is measured, for example, by an electrical resistance probe. Such a probe has a sample element that is exposed to the fluid flow such that particles entrained in the fluid flow may impact the sample element. When a particle impacts the sample element in this way, it may corrode the sample element and therefore change the thickness and hence the electrical resistance of the sample element. An electrical resistance probe therefore measures changes in the electrical resistance of its sample element in order to determine the corrosion rate.
A prior art electrical resistance probe is described, for example, in U.S. Pat. No. 6,693,445 (Sutton). In this patent, a probe is disclosed which is suitable for use with an apparatus for monitoring the corrosion of a material by accurately measuring changes in the resistance of an exposed sample element in relation to a protected reference element. The two elements are electrically connected in series via a bridge. The elements are formed from the same piece of material divided by an elongate slot and are proximate to one another so that the temperature difference between them is minimal. This prevents false indications of corrosion by ensuring that the temperature coefficient of the resistivities is the same in both elements. The reference element is covered with a corrosion-resistant layer. This layer is preferably as thin as possible and also a good thermal conductor to further ensure equal temperature of the reference and exposed elements.
Another prior art electrical resistance detector is described in U.S. Pat. No. 6,946,855 (Hemblade), in which an apparatus is disclosed for monitoring the effect on a material of exposure to a fluid, and thereby monitoring the effect on a section of pipe for carrying the fluid. The apparatus includes a sensor element exposed to the fluid and formed as a ring of the material coaxially mounted within, but electrically insulated from, the section of pipe. Changes in the electrical resistance of the sensor element are monitored. Preferably, the apparatus also includes a reference element electrically insulated from the pipe, electrically connected in series to the sensor element and protected from exposure to the fluid. The elements may both be made from the same material as the pipe and, as they are contained within it, experience the same temperature and pressure variations as the pipe. In this manner a change in the resistance of the sensor element caused by corrosion/erosion by the fluid accurately indicates the degree of corrosion/erosion of the pipe carrying the fluid.
In addition to corrosion, excessive numbers of particles entrained in a fluid may cause blockage of the fluid flow through the conduit. Therefore, it is also important to measure the amount of particulate matter entrained in a given fluid stream and correlate this quantity to corrosion.
In the past, the amount of particulate matter has often been estimated based on the measured corrosion rate. However, some particle impacts may not cause corrosion due to insufficient energy and/or a combination of mechanical properties of both particle and surface, potentially transferring all impulse momentum into the elastic region of the material, hence resulting in no permanent deformation or damage of the impacted surface. Or conversely, particulate matter of low mass but high hardness and sharp edges can cause high corrosion rates. Methods to overcome this have been to use softer element materials but such materials are not representative of the pipe, and therefore cannot be used to measure the extent of actual pipe corrosion. A range of particle size, shape and mechanical properties will potentially give different corrosion rates. In addition the particle velocity will affect the corrosion rate, which is another reason that the particle mass is not easy to determine from the corrosion rate alone. Furthermore, the particle velocity is generally determined from another source, and is not representative of the particle velocity at the sensor itself, which introduces further errors. All these factors undermine the possibility of corrosion measurements alone to determine particle mass flowing through a system.
For this reason, the amount of particulate matter entrained in a fluid stream has previously been calculated using a separate acoustic sensor which measures the acoustic noise on the external surface of the conduit. Such an acoustic sensor is often used in addition to an electrical resistance type probe described above. According to the acoustic sensor technique, as a given particle impacts a surface it will give up some of its kinetic energy in the form of impact energy and will produce a surface acoustic emission. An acoustic sensor is therefore positioned to detect these emissions on the external surface of a pipe. The acoustic emission amplitude and frequency response will depend on a number of variables such as where the sensor is situated (e.g. a bend), the flow regime, gas/oil ratios, trajectory of the entrained particles, number of rebounds, and the extent of internal surface area subjected to impact. However, the acoustic noise measured by such a detector will also be contaminated by flow noise and noises from outside the conduit. Methods are employed to separate flow noise and particle impact noise by the means of measuring the frequency response distribution using selective analogue and digital filtering, but these methods are not perfect. In particular, there can be a close overlap of the acoustic signatures of liquids and solids in some situations. Furthermore, very small particles (i.e. “fines” having a dimension less than 25 microns) will go undetected by such a probe because they tend not to create acoustic signals of sufficient amplitude to be detected. In an attempt to overcome some of these problems, the external acoustic sensor is calibrated in situ by injecting sand to characterise the location. However this is costly and impractical, especially in sub-sea locations and potentially introduces unwanted damage during the calibration process. In order to quantify the amount of sand, the particle velocity is again required, since the acoustic energy will depend on both the mass of the particle and the velocity of the particle at the impacted surface. As described above, the particle velocity is generally determined from another source, and is not representative of the particle velocity at the sensor itself, which introduces further errors.
The present invention aims to address these and other such problems with the art by providing a more accurate and versatile apparatus for monitoring particles in a fluid stream flowing through a conduit.